//合并两个有序链表
//  Definition for singly-linked list.
//  function ListNode(val, next) {
//      this.val = (val===undefined ? 0 : val)
//      this.next = (next===undefined ? null : next)
//  }
//  @param {ListNode} list1
//  @param {ListNode} list2
//  @return {ListNode}

var mergeTwoLists = function(list1, list2){

    if(list1 === null){
        return list2;
    }else if(list2 === null){
        return list1;        
    }else if(list1.val<list2.val){
        list1.next = mergeTwoLists(list1.next,list2);
        return list1;
    }else{
        list2.next = mergeTwoLists(list1,list2.next);
        return list2;
    }
}
